Anatomy and function of the spinal cord and nerves

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Anatomy and function of the spinal cord and nerves

• Spinal cord
• Ch. 14, 17

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Spinal cord is protected by vertebrae, meninges
and CSF

• Canal formed by foramina of vertebrae
• Meninges
• Dura mater
• Arachnoid mater
• Pia mater
• CSF formed in brain
• Found in subarachnoid space

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Anatomy of the spinal cord and spinal nerves
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31 pairs of spinal nerves

• 8 cervical
• 12 thoracic
• 5 lumbar
• 5 sacral
• 1 coccygeal
• All are mixed nerves

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Internal structure of spinal cord

• Roots connect spinal nerves to spinal cord
• Dorsal root- sensory neurons
• Ganglion-cell bodies of sensory neurons
• Ventral root- motor neurons
• Horns- gray matter integrates sensory and motor
  information
• Anterior, posterior, lateral
• White matter- myelinated sensory and motor
  tracts
• Columns- anterior, posterior, lateral

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Regions of white and gray matter

• Dorsal (posterior) horns- sensory neurons
• Ventral (anterior) horns- motor neurons
• Lateral horns- autonomic (motor) neurons
• Columns- ascending (sensory) and descending
  (motor) tracts

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Sensory and motor tracts

• Named according to its position and pathway
• Sensory impulses move up tracts and columns
• Join the pool of sensory information
• Voluntary motor output emanates from cerebral
  cortex (direct pathways)
• Involuntary output originates in brain stem and
  hypothalamus (indirect pathways)

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Spinal nerves

• Comprise the peripheral nervous system (PNS)
• Connect the CNS to muscles, sensory receptors and
  glands
• 31 pairs of spinal nerves
• Posterior and anterior roots (mixed nerves)

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Distribution of spinal nerves

• Branches (rami)
• Posterior (muscles and surfaces anterior)
• Meningeal branch (CNS)
• Rami communicantes- autonomic system
• Plexuses- network of axons
• Cervical, brachial, lumbar, sacral

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Map of spinal nerves

• Motor fibers

• Sensory fibers

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Major plexuses example cervical plexus

• Origin (C1- C5)
• Superficial or deep
• Superficial- skin mostly sensory
• Deep-muscle largely motor
• Severing spine above phrenic nerve causes
  respiratory arrest

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More plexuses

• Brachial
• Shoulders, upper limbs
• Axial, musculocutaneous, radial, medial, ulnar
• Lumbar
• Anterolateral abdominal wall, genitalia, part of
  lower limbs
• Sacral and coccygeal
• Buttocks, perineum, lower limbs
• Damage can cause loss of sensation, palsy, loss
  of motor control
• Intercostal nerves directly innervate muscles (no
  plexuses)

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Dermatomes

• Sensory neurons convey information form skin to
  CNS
• Dermatomes- areas of skin that provide sensory
  input to a particular pair of nerves
• Some overlap
• Can be useful for diagnosis

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Cervical plexus
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Brachial plexus
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Brachial plexus (C1-C8, T1)
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Lumbar and sacral plexuses
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Reflex arcs- fast involuntary responses to stimuli
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Reflex arcs may be somatic or autonomic

• Patellar reflex (knee-jerk)
• Tap patellar ligament quadriceps femoris
  contracts
• Achilles reflex (ankle-jerk)
• Tap calcaneal tendon gastrocnemius and soleus
  muscles contract
• Babinski sign- test for continuity of the
  corticospinal tract (goes from positive to
  negative with development)
• Abdominal reflex
• Can be useful for diagnosis of chronic disease or
  nerve damage

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Types of reflexes
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Autonomic vs. somatic nervous system

• Somatic senses (pain, thermal, tactile,
  proprioreceptive)
• Somatic motor neurons- skeletal muscle
• Usually voluntary
• Motor units
• Acetylcholine

• Autonomic sensory (chemoreceptors, mechano-
  (stretch) receptors
• Motor neurons regulate visceral activities can
  function independently of ANS
• Preganglionic and postganglionic neurons
• Acetylcholine or norepinephrine

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Organization of ANS
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Different types of ganglia

• Sympathetic
• Sympathetic trunk
• Innervate organs above the diaphragm
• Superior, middle, inferior cervical ganglia
• Prevertebral
• Below the diaphragm
• Celiac, superior and inferior mesenteric ganglia
• Parasympathetic
• Preganglionic axons are longer than in
  sympathetic ganglia
• Preganglionic neurons are myelinated,
  postganglionic neurons are not

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Review of spinal nerve distribution
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Sympathetic (thoracolumbar)

• Sympathetic chain ganglia, paired
• Head and thoracic cavity
• Collateral ganglia, unpaired
• Abdominopelvic cavity
• Suprarenal medullae, paired
• hormonal

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Functions of the sympathetic division

• Sympathetic chain ganglia
• Diversion of blood flow to skeletal muscles and
  brain
• Sweating pupil dilation
• Collateral ganglia
• Vasoconstriction restrict urinary function
• Suprarenal medullae
• Release of epinephrine and norepinephrine

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Parasympathetic (craniosacral) division of ANS
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Parasympathetic system

• Ganglia are located near, or in, target organs
• Effects are directed more toward specific target
  organs

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Autonomic plexuses along major arteries
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Regulation by dual innervation

• Systems intersect in plexuses
• Cardiac
• Pulmonary
• Esophageal
• Celiac
• Inferior mesenteric
• Hypogastric

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Postganglionic neurons

• Sympathetic system
• One presynaptic neuron can diverge into many
• Many organs can be affected at once (divergent)
• May extend to adrenal medullae
• Parasympathetic system
• Many presynaptic neurons can converge on a single
  effector
• Effect can be localized to a single effector

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Action by neurotransmitters in ANS

• Acetylcholine (cholinergic neurons)
• All preganglionic
• Sympathetic postganglionic innervation of sweat
  glands
• All parasympathetic postganglionic neurons
• Nicotinic, muscarinic receptors
• Norepinephrine (adrenergic)
• a and ß receptors
• Can be excitatory or inhibitory

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Functions of ANS

• fight or flight (sympathetic)
• More widespread and longer lasting
• Norepinephrine and epinephrine can act as
  hormones as well as neurotransmitters
• rest and digest (parasympathetic)
• Salivation, lacrimation, urination, digestion and
  defecation

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Control of ANS
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Control of autonomic functions

• Reflexes
• Blood pressure
• Digestion
• Defecation and urination
• Control within brain
• Brain stem (cardiovascular, swallowing ,
  digestion)
• Spinal cord (elimination)
• Control and integration center is hypothalamus

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Sympathetic vs parasympathetic effects

• Parasympathetic tends to be organ-specific
• More reflexes
• Sympathetic tends to be more holistic
• Divergence
• Hormonal distribution